1. Field of the Invention
The invention relates generally to the field of production of hydrocarbons from wellbores. More specifically, the present invention relates to a method and apparatus to evaluate the integrity of bonds that adhere wellbore casing to a wellbore. Yet more specifically, the present invention relates to a method and apparatus capable of evaluating bond integrity of bonding cements of different densities. Yet even more specifically, the present invention relates to a method and apparatus capable of evaluating cement bond integrity that can distinguish a casing bonded with a light weight cement from a casing surrounded with a micro-annulus fluid.
2. Description of Related Art
Hydrocarbon producing wellbores comprise casing 8 set within the wellbore 5, where the casing 8 is bonded to the wellbore by adding cement 9 within the annulus formed between the outer diameter of the casing 8 and the inner diameter of the wellbore 5. The cement bond not only adheres the casing 8 within the wellbore 5, but also serves to isolate adjacent zones (Z1 and Z2) within the formation 18 from one another. Isolating adjacent zones can be important when one of the zones contains oil or gas and the other zone includes a non-hydrocarbon fluid such as water. Should the cement 9 surrounding the casing 8 be defective and fail to provide isolation of the adjacent zones, water or other undesirable fluid can migrate into the hydrocarbon producing zone thus diluting or contaminating the hydrocarbons within the producing zone.
To detect possible defective cement bonds, downhole tools 14 have been developed for analyzing the integrity of the cement 9 bonding the casing 8 to the wellbore 5. These downhole tools 14 typically include transducers 16 disposed on their outer surface capable of emitting acoustic waves into the casing 8 and recording the attenuation of the acoustic waves as they travel, or propagate, across the surface of the casing 8. By analyzing the attenuation of the acoustic wave, the efficacy and integrity of the cement bond can be evaluated.
The amount of attenuation however can depend on how the acoustic wave is polarized and coupling condition between the casing 8 and the cement 9 bonding the casing 8 to the wellbore 5. Typical downhole tools 14 having acoustic wave transducers 16 generate acoustic waves that are polarized perpendicular to the surface of the casing 8. Such waves are referred to as compression/shear or P-SV waves since the particle motion direction of either compressional (P) or shear (S) component of the acoustic wave is in a vertical (V) plane perpendicular to the casing 8. The attenuation of the acoustic wave as it propagates along the surface of the casing 8 varies in response to the condition of the cement bond and also in response to the type of cement 9 disposed between the casing 8 and the formation 18. More specifically, as the acoustic wave propagates along the length of the casing 8, the wave loses, or leaks, energy into the formation 18 through the cement bond—it is this energy loss that produces the attenuation of the acoustic wave. Conversely, when the casing 8 is not bonded, a condition also referred to as “free pipe,” the micro-annulus fluid behind the casing does not provide for any shear coupling between the casing 8 and the formation 18. Loss of shear coupling significantly reduces the compressional coupling between the casing 8 and the formation 18. This result occurs since fluid has no shear modulus as well as a much lower bulk modulus in relation to cement. Because of these physical characteristics of fluid, the entire SV component of the P-SV wave and a large portion of the P component of the P-SV wave do not propagate outside of the casing 8 and thus experience a much reduced attenuation.
Reduced attenuation of an acoustic wave is not limited to situations where the casing 8 is surrounded by fluid, but the presence of some cements can also significantly reduce acoustic wave attenuation. For example, light weight cement (LWC), or cement having a density less than approximately 12 lbs/gal can reduce acoustic wave attenuation. Light weight cement has a shear modulus, thus light weight cement can maintain shear coupling between the casing 8 and the formation 18. However, the density of light weight cement is not substantially greater than the density of many fluids (such as water), thus the attenuation of some acoustic waves, especially P-SV waves, is diminished when encountering casing 8 surrounded by a light weight cement. The result of this reduced attenuation is a decreased ability to detect the difference between fluid and light weight cement as well as a diminished capacity to detect poor cement bonds in light weight cement.
Therefore, there exists a need for a device and method to conduct cement bond logs within a casing, where the device and method is capable of differentiating between fluid that surrounds a casing and light weight cement bonding a casing.